Deoxycytidine kinase enzyme activity and deoxycitidine kinase mRNA content were determined at different positions of the cell cycle in permanent cell lines. There was no variation of deoxycytidine kinase activity during the cell cycle in two cell lines, whereas in two other lines the enzyme activity was induced (10- and 15-fold) at the G1/S boundary. In contrast, the level of deoxycytidine kinase mRNA never displayed any cell cycle-dependent changes. The decay of enzymatic activity was measured after addition of cycloheximide in different phases of the cell cycle; the enzyme was much more stable in cells with constant activity. We suggest that post-transcriptional mechanisms account for the periodic behaviour of the enzyme activity, and that whether this regulation can be detected depends on the half-life of deoxycytidine kinase.
